1. Field of the Invention
This invention relates to a wiper driving apparatus, and more particularly to improvements in a device for stopping a wiper at a predetermined position, effectively utilized, for example, in a wiper driving apparatus mounted on vehicles such as automobiles which may run in an area of snowfall.
2. Related Art Statement
In general, in a wiper driving apparatus mounted on a vehicle such as an automobile, in order to secure the safety driving, there is provided a device for stopping a wiper at a predetermined position, by which a wiper blade is moved to a bottom corner of a window and stopped thereat, even when a switch at hand is turned off at random.
A conventional device of this type for stopping a wiper at a predetermined position is constructed such that an electric conductor plate including a non-conductive portion in part has been attached to an end face of a drive gear, and a contact is provided in a housing, so as to slide on the electric conductor plate in accordance with rotation of the drive gear, the contact and the electric conductor plate constituting a switch for turning on and off an electric circuit for a motor.
When the switch at hand is turned off at random, an electric circuit for driving a wiper blade to the predetermined position is formed passing through the electric conductor plate and the contact. Thereafter, when the wiper blade arrives at the predetermined position, a main circuit is cut off at the non-conductive portion. At the same time, an armature circuit is short-circuited, whereby electric breaking is applied. With these operations, a wiper motor is automatically stopped at the predetermined position.
However, in the wiper driving apparatus having the above-described device for stopping the wiper blade at a predetermined position, when an external force is applied in a direction opposite to the normal rotation of the drive gear, for example, in the case where snow is scraped together to the bottom of the window, the chattering phenomenon, in which conduction and non-conduction are repeated between the electric conductor plate and the contact, occurs. When the chattering occurs, the wiper driving apparatus is heated to a considerably high temperature, and is thereby damaged in quality and durability. This is a problem inherent in the conventional wiper driving apparatus.
In this circumstances, there has been developed following wiper driving apparatuses which can prevent the chattering phenomenon. (Refer to Japanese Utility Model Laid-Open Nos. 61-21657, 61-191960 which corresponds to U.S. Pat. No. 4,700,026, 62-176068 and 2-41857 which corresponds to U.S. Pat. No. 5,068,500).
That is, in these wiper driving apparatuses, a clutch member is rotatably provided for rotating together with a drive gear in a direction that the drive gear is biased to. A switch mechanism for turning on and off the electric circuit of the motor, which consists of electric conductor plates and the contacts, is interposed between the clutch member and a housing.
During normal operation, the electric conductor plates and the contacts are rotated relatively with each other by the clutch member along with the rotation of the drive gear. When an external force to rotate the drive gear reversely is applied to the drive gear, the electric conductor plates and the contacts are not caused to follow the reverse rotation of the drive gear due to the clutch member, so that the relative backward movement between the electric conductor plates and the contacts can be prevented.
However, these wiper driving apparatuses cannot be rendered compact in size satisfactorily.
For example, in the conventional apparatus, an insulator is used in the clutch member and the clutch member is rotatably provided on a supporting axle of the housing, so that the housing must be formed to increase its height.
Furthermore, the electric conductor plates are disposed coaxially around a clutch plate, so that the housing must be increased in diameter and in height in order to constitute therein the switch turning on and off the electric circuit of the motor.